Concrete is strong in compression, but relatively weak in tension. In most structures, such as buildings or bridges, forces such as wind, gravity, and earthquakes subject the structure, and therefore, the concrete to compressive and tensile stresses. Steel reinforcement is typically used to resist the tensile forces within a concrete structural member. The steel reinforcement is placed in the form before the concrete is poured within the tension zone. The steel reinforcement is designed to work with the concrete to resist tension forces and to control concrete cracking. For additional efficiency and economy, steel reinforcement, such as prestressing steel, can be stressed before the building forces are applied to the concrete member in the opposite direction of the to-be-applied force. Such stressing counteracts the applied force and allows the use of less steel reinforcing.
There are two basic methods of stressing concrete before the concrete is subjected to design forces (i.e., prestressing): post-tensioning and pre-tensioning. Pre-tensioned prestressed concrete is usually fabricated at a plant remote from the final construction site. In that case, the steel tendons are stressed before the concrete is placed. With post-tensioned prestressed concrete, steel tendons are stressed after the concrete has been placed and gained sufficient strength at the construction site. The present invention is primarily concerned with post-tensioned prestressed concrete.
With post-tensioned prestressed concrete, after the concrete has hardened, a hydraulic jack is used to pull strands of steel or tendons encased in the concrete. The prestressing steel strands are separated from the concrete by polyethylene sheathing. This sheathing allows the encased steel move with respect to the concrete. Thus, the steel can be stressed without frictional resistance from the concrete which effectively eliminates tensile stresses in the concrete due to the prestressing. The tension force in the steel is then permanently transferred to the concrete as a compressive force through anchoring devices at the end of the concrete member.
In post-tensioned concrete, the concrete is simply poured over the unstressed steel strands or tendons. Initially, therefore, both concrete and steel have no stress. As the concrete cures or hardens, it gains its compressive strength. After about 24 hours, it usually has reached about 75 percent of its full design strength. It is at this point when the steel is stressed. The steel is then stressed to high stresses, (e.g., 216,000 psi) but within its elastic limit. While stressed, the steel is then permanently attached to anchors at the ends of the concrete beam. Thus, the steel is in tension (i.e, pulling on the anchors) while the concrete remains in compression because both anchors are pushing on the concrete. The result is a considerable reserve of compressive stress at the bottom of the beam that in turn can be used to counteract the tensile stresses from an applied load.
Before the concrete is poured, anchorages are set and attached to the form. Once the concrete has hardened, the anchorages are embedded into the concrete. Typically, the anchorages are not flush with the concrete edge. Pocket formers are used to separate the anchorages from the edge of the concrete. Thus, after the concrete has hardened, these pocket formers leave anchorage cavities in the concrete. Anchorage cavities have tapered wedge-receiving seats and passages through which the tensioning cables extend. FIG. 1 is a cross-section of an end of a concrete member 100 through anchorage 105 and anchorage cavity 102. Steel strand 104 extends from polyethylene sheathing 106 through the center of anchorage 105 and anchorage cavity 102.
A stressing ram (not shown) is used to pull the tendon 104 to the correct pressure and elongation. Then, anchoring wedges 108a and 108b are driven between tendon 104 and the surrounding anchorage 105. The wedges grip tendon 104 as the jack releases it. Thus, the wedges 108a and 108b keep tendon 104 from slipping back through anchorage 105 into the concrete. Once the all of the tendons have been properly stressed, the ends are cut, the pockets are grouted, and the beam ends are encapsulated to protect the tendons from corrosion.
Wedges 108a and 108b are pieces of tapered metal with teeth that bite into the prestressing tendon during transfer of the prestressing force. The teeth are beveled at the front end to ensure gradual development of the tendon force over the length of the wedge. Two piece wedges are normally used for monostrand tendons. Proper installation of the wedges into the concrete is critical. The wedges must be evenly spaced and exactly fitted into anchorage 105. If the wedges are unevenly spaced or not properly fitted, the wedges could slip and ruin the entire prestressing operation.
It has been found that it is difficult to properly align, install and seat the wedges. The wedges must be installed into the back surface 103 of anchorage cavity 102 which has limited access. Furthermore, both wedges need to be equally spaced and driven into anchorage 105 by an equal amount. Currently, a small hand held device is used to install the wedges in the anchorage cavity. This small hand held device consists of a bent longitudinal bar. The user positions one wedge into the space between anchorage 105 and protruding tendon 104. Then with the other hand, slides the hand held device along tendon 104 with enough force to partially anchor the wedge into anchorage 105. The user then repeats the procedure for the other wedge on the opposite side of the strand. The tool only allows for partial installation of one wedge at a time. Thus, the user must alternate seating the wedges on both sides of the cable constantly to ensure that the wedges are evenly aligned as they enter their respective wedge seats. This process is time consuming, inefficient and difficult due to the limited space available in the anchorage cavity.
What is needed, therefore, is an efficient and cost-effective device to place the anchorage wedges into the anchorages prior to tensioning the monostrand members.